1. Field of the Invention
The present invention relates to a mark detecting device, an adjustment method, and a printing apparatus.
2. Description of the Related Art
As a printing system that forms images on both surfaces of a web, a printing system in which two printing apparatuses are connected in tandem has been widely known. In the printing system, a first printing apparatus P1 prints an image on the front surface of the web, the web having the printed front surface is reversed, and a second printing apparatus P2 prints an image on the rear surface of the web. In general, a strip-shaped sheet having feed perforations at both ends has been known as the web. After a printing process, the perforations at both ends of the sheet need to be cut out and discarded. Therefore, a printing system that can correspond to a web without a perforation has been widely spread.
In the printing system, when the web without feed perforations is transported, a friction transport method using a drive roller is used, unlike a feed perforation transport method using a tractor. Therefore, there is a variation in the amount of transport of the web due to a slip occurred between the drive roller and the web during friction transport or the tolerance of the drive roller. In addition, in tandem printing, the web transported to the second printing apparatus P2 is likely to be expanded or contracted after passing through the first printing apparatus P1. For example, in an electrophotographic printing apparatus, since toner is heated and fixed to the web, the web is thermally contracted.
A printing system has been proposed in which the second printing apparatus P2 detects a print position on the front surface and a speed control device corrects the print speed of the second printing apparatus P2 on the basis of the detection timing, in order to accurately align the print positions on the front and rear surfaces, regardless of a variation in the amount of transport of the web during friction transport or the expansion or contraction of the web during tandem printing (Japanese Patent No. 3680989).
In addition to double-side printing, when a pre-printed web is printed, the alignment between the print positions is performed. The color, position, and size of a pre-printed mark differ depending on web. Therefore, a mark detecting device has been proposed which is movable to any position in the horizontal direction (main scanning direction) and can detect the background and the pre-printed mark of any color using a reflective optical sensor, which is a representative example of a detecting device using a light emitting element composed of a plurality of red (R), blue (B), and green (G) light sources (Japanese Patent Application Laid-Open (Japanese Translation of PCT Application) No. 2001-522742).
It is preferable that the web printing apparatus align the print positions for various kinds of webs. However, in the mark position detection technique according to the related art, for example, when a glossy sheet is used as the web, in some cases, the detection accuracy of the mark is reduced and it is difficult to align the print positions. When using a normal non-glossy (matte) sheet, a sensor receives light that is diffusely reflected from the surface of the sheet, and the mark and the background are discriminated on the basis of a variation in the amount of light received. An example in which the reflective optical sensor detects the mark when a matte sheet is used as the web is shown in FIGS. 4A1 to 4C1. For example, when the sensor faces the mark (black), the amount of light received by the sensor is small. When the sensor faces the background (white), the amount of light received by the sensor is large. The sensor has a function of storing received light amount data L1 when facing the mark and received light amount data L2 when facing the background and a function of setting an intermediate value L0 as a threshold value. In the sensor, for example, when the amount of light received is more than L0, a detection signal becomes an “H” level, while when the amount of light received is less than L0, the detection signal becomes an “L” level. At the timing when the mark (black) passes through the sensor, the detection signal becomes an “L” level, and information indicating the passage of the mark is converted into an electric signal. In this way, it is possible to detect the mark.
In contrast, in the case of a glossy sheet, such as a coating sheet, the sensor receives light that is totally reflected from the surface of the glossy sheet, similar to specular reflection, in addition to diffusely reflected light. An aspect in which the reflective optical sensor detects the mark when a glossy sheet is used as the web is shown in FIGS. 4A2 to 4C2. In the case of the glossy sheet, the amount of light totally reflected is more dominant than the amount of light diffusely reflected. Since the amount of light received by the sensor during total reflection does not vary depending on whether light is reflected from the mark (black) or the background (white), the difference between the received light amount data L1 when the sensor faces the mark and the received light amount data L2 when the sensor faces the background is small. The detection distance between the sensor and the web vary depending on the thickness of the web used or a variation in transport to thereby vary the received light amount data. Therefore, in some cases, the variation ranges of the received light amount data L1 when the sensor faces the mark and the received light amount data L2 when the sensor faces the background exceed the threshold value L0, and it is difficult to discriminate the mark and the background. As a result, even when the mark passes through the optical axis of the sensor, it is difficult to detect the mark and determine the position. Therefore, a print error occurs and printing stops, which results in a reduction in the operation rate of the printing apparatus.